1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of manufacturing a semiconductor device in which a gate insulating film of a high dielectric constant material is used in a MOSFET.
2. Description of the Related Art
FIG. 16 is a schematic perspective view of nMOSFET formed on a semiconductor substrate. As shown in FIG. 16, n-type source/drain regions 101 and 102 are formed in a p-type silicon semiconductor substrate 100. A gate insulating film 103 and a gate electrode 104 are sequentially formed on the silicon semiconductor substrate 100 in such a manner that the gate insulating film 103 and the gate electrode 104 are located between the n-type source/drain regions 101 and 102. Usually, polycrystalline Si, polycrystalline SiGe, metal or the like is used as a gate electrode material.
As the MOSFET is miniaturized, the gate insulating film is required to be more reduced in thickness. As the gate insulating film, hitherto, a silicon dioxide film or silicon oxynitride film has been used. However, since the direct tunneling current increases due to the reduction of the film thickness, the limit of the film thickness reduction is about 2 nm in the silicon dioxide film and silicon oxynitride film. In the circumstances, it has been proposed to apply a metal silicate film to the gate insulating film. Since the metal silicate film is higher in dielectric constant than the silicon dioxide film and in crystallization temperature than a metal oxide film, it matches with the conventional MOSFET process using poly-Si or poly-SiGe gate electrodes. Further, to suppress boron peratration from the gate electrode, it has been also proposed to apply a nitrided metal silicate film to the gate insulating film (Jpn. Pat. Appln. KOKAI Publication No. 2000-49349, column 10, and FIG. 8). However, satisfactory interface characteristic is not obtained. Metal nitride is conductive, and thus it is high in leakage current and in electric charge trap density. Further, a metal silicide film is formed between the metal silicate film and the gate electrode to degrade the insulation therebetween.
In a process of forming a gate insulating film made of nitrided metal silicate, for example, Hf(Zr)SiON, when a Hf(Zr)SiO film is nitrided by using nitrogen plasma, a surface of a silicon semiconductor substrate is also nitrided, so that the interface state density is increased and thus the mobility of electric carriers is lowered. To prevent the nitridation of the silicon substrate surface, it has been proposed to form a silicon dioxide film layer or silicon oxynitride film layer on the silicon substrate surface before depositing the Hf(Zr)SiO film. However, in this case, a fixed charge is generated at the interface between the silicon dioxide film layer or silicon oxynitride film layer and the Hf(Zr)SiO film, so that the mobility of carriers is not improved.
When, as is conventional, the gate insulating film is formed of the silicon dioxide film or silicon oxynitride film, the film thickness reduction of the gate insulating film associated with the miniaturization trend of a MOSFET is close to its physical limit, and it is nowadays considered inevitable to use a high dielectric constant material gate insulating film having a higher dielectric constant than the silicon dioxide film or silicon oxynitride film. It is, however, a problem of such a high dielectric constant material is that the thermal stability is poor, and it has been proposed to introduce nitrogen to improve the thermal stability. To this end, it is typical to make the high dielectric constant material into the nitride by using plasma or NH3. However, in the method of introducing nitrogen, the surface of the silicon semiconductor substrate is also nitrided, so that the interface state is increased, and the mobility of carriers deteriorates. To prevent nitridation of the silicon semiconductor substrate, it is advantageous to provide a silicon dioxide film or acid nitride film of 1 nm or less between the high dielectric constant material and the silicon substrate, however, this generates fixed charges at the interface between this interfacial layer and the high dielectric constant material, and the mobility of carriers is not improved.